I would use Wurth body wax. It comes in quart containers but you need a special gun with wands to apply it. It's leaves a nice wax film on the inside of the panels to keep moisture out. Many european car companies use it from the factory. Great stuff. I can try to find out where you can get it if you are interested. I have the specific gun to use for it. If you would like to borrow it I would have no problem with that. I'm in Saline, just ourside AnnArbor. It can be messy to apply though and you need and air source. 3M also makes a similar wax, called Rustfighter I but it's not as thick as the wurth.